The invention is directed to a process
the production of substantially isotropic, spherical fuel or absorber elements of higher strength for high temperature reactors by pressing a mixture of graphite molding powder containing binder resin with coated nuclear fuel or absorber particles to form a spherical nucleus, pressing a shell made of the same graphite molding powder, carbonizing the binder resin and vacuum calcining up to about 2000.degree. C.
Spherical fuel or absorber elements for high temperature reactors consist of a spherical ball nucleus, in which the nuclear fuel or the absorber material is present in the form of coated nuclear fuel or absorber particles, embedded in a graphitic matrix, as well as an outer fuel or absorber free shell which encloses these spherical nuclei and is made of the same material as this graphitic matix.
The elements are produced by molding as is described e.g. in German Pat. No. 1909871 whereby first the spherical nucleus is preliminarily molded from a mixture of so-called graphite molding powder and the coated particles and subsequently using this graphite molding powder the fuel or absorber free shell is molded on this nucleus. Subsequently the binder of the graphite molding powder is carbonized.
It is required of spherical fuel or absorber elements that the physical or mechanical properties of the matrix graphite are substantially isotropic. As a measure for a possible anisotropy there is defined customarily the quotient of the thermal coefficient of expansion measured parallel and perpendicular to specific preferred directions. Such anisotropy is produced because graphite normally is a highly anisotropic material because of its crystal structure, which only shows macroscopic isotropic properties if there is produced a random statistical distribution of the graphite granules of the crystallites.
Insofar as they are not carried out most substantially isostatically, molding processes represent particularly techniques which can lead to a preferred orientation of the graphite granules corresponding to the preferred direction of the molding process and thus to a significant anisotropy of the molded body produced.
This state of affairs, anisotropic properties of the graphite particles on the one hand and the requirement of isotropic properties of the elements produced from these graphite granules on the other hand has led to the molding of spherical fuel or absorber the elements today exclusively by nearly isostatic molding procedures. Thereby the spherical nucleus if preliminarily pressed in an elastic mold, as a rule made of silicone rubber material, likewise the shell is molded in a mold which likewise normally consists of silicone rubber. Through their elasticity these molds cause a preeminently radial distribution of pressure and therefore in the molding lead to the desired statistical distribution of the graphite granules so that the resulting molded article exhibits the most nearly isotropic properties.
On the other hand, the use of those molded shapes constrains the molding process to be carried out a lower temperatures, as a rule at room temperature because of insufficient temperature resistance, through which high molding pressure are required.
In order to prevent the danger of the decomposition of the coated particles arising at high molding pressures there are necessary expensive encasing processes such as described e.g. in German Pat. No. 1909871 or specially constructed molding compositions such as described, e.g. in German Pat. No. 2348282 (and related Huschka U.S. Pat. No. 3,978,177, the entire disclosure of which is hereby incorporated by reference and relied upon). Furthermore, the high molding pressure limits the life of the compression mold. Independent of these problems related to the molding pressures it is necessary to shape the surface of the spherical nucleus in order that there is formed a sufficient bond between the spherical nucleus and the pressed on spherical shell which prevents the spherical shell from spalling from the spherical nucleus either during the necessary concluding heat treatment step of the fuel or absorber element, i.e., the necessary coking of the binder and a subsequent purification and degassing, or during the mechanical stress in the reactor operation. This shaping of the spherical nucleus surface, however, requires complicated molding tools.
Therefore, it was the problem of the present invention to develop a process for the production of substantially isotropic, spherical fuel or absorber elements of high strength for high temperature reactors by molding a mixture of graphite molding powder containing a binder resin with coated nuclear fuel-or absorber particles to form a spherical nucleus, pressing on a shell made of the same graphite molding powder, carbonizing the binder resin and vacuum calcining up to about 2000.degree. C. whereby there should be eliminated elastic compression molds, a special shaping of the spherical nucleus should be unnecessary and should be preliminary molded at higher temperatures.